Syntrophy between fermentative and purple phototrophic bacteria for carbohydrate-based wastewater treatment

ABSTRACT Fermentative chemoorganoheterotrophic bacteria (FCB) and purple photoorganoheterotrophic bacteria (PPB) are two interesting microbial guilds to process carbohydrate-rich wastewaters. Their interaction has been studied in axenic pure cultures or co-cultures. Little is known about their metabolic interactions in open cultures. We aimed to harness the competitive and syntrophic interactions between PPB and FCB in mixed cultures. We studied the effect of reactor regimes (batch or continuous, CSTR) and illumination modes (continuous irradiation with infrared light, dark, or light/dark diel cycles) on glucose conversions and the ecology of the process. In batch, FCB outcompeted (>80%) PPB, under both dark and infrared light conditions. In CSTR, three FCB populations of Enterobacteriaceae, Lachnospiraceae and Clostridiaceae were enriched (>70%), while Rhodobacteraceae relatives of PPB made 30% of the community. Fermentation products generated from glucose were linked to the dominant FCB. Continuous culturing at a dilution rate of 0.04 h -1 helped maintain FCB and PPB in syntrophy: FCB first fermented glucose into volatile fatty acids and alcohols, and PPB grew on fermentation products. Direct supply of carboxylates like acetate under infrared light enriched for PPB (60%) independent of reactor regimes. Ecological engineering of FCB- and PPB-based biorefineries can help treat and valorize carbohydrate-based waste feedstocks. GRAPHICAL ABSTRACT